Funds to update the Duke Research Single Photon Emission Computed Tomograph (SPECT) are requested. The objective of this update is to provide a unique, state-of-the-art research instrument that has the capability to meet the scientific needs and enhance the future research endeavors of the major users. The DUKE-SPECT system has been used exclusively for SPECT research for over 9 years. The proposed update will be the first comprehensive upgrade and will solve specific deficiencies that limit the system's performance. The update will consist of 1) replacing the old, and electronically unstable, scintillation detector heads with new detectors, and 2) replacing the obsolete processor having limited memory capability with a dedicated current-generation processor to meet the needs of the users for data acquisition, reconstruction, and analysis. The first user's (2R01CA33541) long term objectives include the assessment of the quantitative accuracy of SPECT and the development of new compensation approaches to improve SPECT quantification. Major research projects include Monte Carlo modeling and 3-D cone beam collimation. The present processor is not capable of adequately reconstructing high resolution cone beam data. The objective of the second user (1T32HL07503) is to support research and training of physicians. Its needs relating to accurate SPECT quantification will be enhanced by the update. The research of the third user (1R23CA39251) involves the development of a unified reconstruction algorithm using Maximum Likelihood methods. The update will provide accurate projection data and offers the potential to implement this algorithm in the research unit. The research proposal of the fourth user (R01CA43638-01) relates to the localization of radiolabeled monoclonal antibodies in animals and patients. Quantitative SPECT measurements are an important component of this research. The proposed update provides the only available alternative that will simultaneously support the major user group, and would allow new research activities to be performed that otherwise would not be possible.